SUMMARY

This report documents the first use of magnetic resonance images (MRIs) of
living dolphins to register functional brain scans, allowing for the
exploration of potential mechanisms of unihemispheric sleep. Diazepam has been
shown to induce unihemispheric slow waves (USW), therefore we used functional
imaging of dolphins with and without diazepam to observe hemispheric
differences in brain metabolism and blood flow. MRIs were used to register
functional brain scans with single photon emission computed tomography (SPECT)
and positron emission tomography (PET) in trained dolphins. Scans using SPECT
revealed unihemispheric blood flow reduction following diazepam doses greater
than 0.55 mg kg-1 for these 180-200 kg animals. Scans using PET
revealed hemispheric differences in brain glucose consumption when scans with
and without diazepam were compared. The findings suggest that unihemispheric
reduction in blood flow and glucose metabolism in the hemisphere showing USW
are important features of unihemispheric sleep.

Functional scans may also help to elucidate the degree of hemispheric
laterality of sensory and motor systems as well as in neurotransmitter or
molecular mechanisms of unihemispheric sleep in delphinoid cetaceans. The
findings also demonstrate the potential value of functional scans to explore
other aspects of dolphin brain physiology as well as pathology.

Only once have investigators explored hemispheric physiology beyond
recording EEG and other electrophysiological signs. The study of Koval'zon and
Mukhametov was aimed at determining if brain temperature cycled with SWS
(Koval'zon and Mukhametov,
1982). The authors studied four Black Sea bottlenose dolphins
(Tursiops truncatus) and one harbor porpoise (Phocoena
phocoena). Two thermisters were implanted in each animal - one in the
auditory cortex of each cerebral hemisphere. During SWS, the temperature of
the hemisphere displaying SWS was ∼1°C lower than the opposite
hemisphere, which displayed an EEG indistinguishable from the waking state.
Koval'zon and Mukhametov concluded that a unihemispheric reduction of
metabolic heat produced by neurons and glia accompanied the SWS
(Koval'zon and Mukhametov,
1982).

Later, Mukhametov noted that the benzodiazepine tranquilizer diazepam
induced `dolphin unihemispheric SWS in its most vivid form'
(Mukhametov, 1987). Diazepam
binds to GABAA receptors and a change in the sensitivity of
GABAA receptors is one mechanism that might be involved in dolphin
unihemispheric SWS. There is ample evidence that GABA plays a major role in
sleep regulation in land mammals (Ali et
al., 1999; Xi et al.,
1999; Gallopin et al.,
2000; Koop et al.,
2004). Garey et al. (Garey et
al., 1989) determined that the quantitative distribution of GABA
neurons in the Black Sea porpoise (Phocoena phocoena) within the
visual cortex is similar to that in land mammals.

It can be said that bottlenose dolphins and their close relatives in the
cetacean family, Delphinidae, have large brains and have reached the zenith of
cetacean brain development (Marino,
1998; Ridgway,
1999; Marino et al.,
2004). Modern morphomolecular studies of fixed material have begun
to reveal information relative to the neurochemistry of some regions of the
dolphin brain (cf. Hof et al.,
1995; Glezer et al.,
1998; Manger et al.,
2003; Manger et al.,
2004). However, non-invasive means of investigating this large and
highly organized brain in the living animal have been quite limited and there
is little understanding of the neurotransmitter and neuromodulator
distribution in the dolphin brain as a whole. Prior to our recent studies
(Houser et al., 2004), live
cetacean scans were limited to one computed tomography CT) study of a pygmy
sperm whale with a sinus abscess (Tristan
et al., 2001). Houser et al.
(Houser et al., 2004) expanded
the use of medical imaging modalities on live cetaceans to include functional
scanning (SPECT and PET) and coupled the images obtained with these scans to
structural imagery obtained via CT. To investigate brain function in
context of the finer anatomy of the brain, CT imaging of dolphin anatomy must
be replaced by an imaging modality sensitive to soft tissue. MRI permits
detail of soft tissues to be discerned, but the application of MRI to living
cetaceans has yet to be reported.

A trained dolphin slides out of the water onto a padded transport mat. (A)
Dolphin swims around its bay enclosure. (B) The dolphin is signaled to station
in front of the trainer. (C) The dolphin slides out onto the padded transport
mat. (D) The padded sides of the transport mat are brought together so that
the dolphin is secure in the mat with the lateral walls up and fastened.

The combination of functional imaging with soft and hard tissue structural
imaging will permit in vivo assessments of dolphin brain functional
anatomy. The information obtained from such scans will yield invaluable
information on dolphin brain physiology, making possible the understanding of
some of the apparently distinctive capabilities of dolphins. Such capabilities
include their excellent SONAR system, the tactile sensitivity of their skin,
the ability of the brain to withstand hypoxia during diving, acoustic
communication, underwater vision, and how dolphins sleep at sea. Additionally,
the combined imaging modalities can increase both our understanding of how
various medications affect brain chemistry and our ability to employ imaging
techniques in the diagnoses of illness in the dolphin brain.

Here we report results of the first functional scans of the dolphin brain
registered to MR images obtained in the same animals. The functional scans,
SPECT and PET, were collected with and without the administration of diazepam
to induce SWS. SPECT scans were used to monitor cerebral blood flow and PET
scans were used to estimate brain glucose metabolism via the uptake of a
glucose analog. Differences in treatment and non-treatment scans were used to
describe the physiology of unihemispheric SWS as a function of the brain's
specific anatomy by co-registration to MRI scans. The results provide the
first ever indication of localized and regional variations in brain metabolism
and blood flow resulting from the induction of unihemispheric SWS.

Materials and methods

Procedures for scans

Three live, adult males (WEN, OLY and FLP) and one post mortem,
adult male (MAY) bottlenose dolphins (Tursiops truncatus Montagu)
were used in this study (Table
1). This study includes two MRIs (WEN and MAY), three SPECT scans
(2 WEN and 1 FLP), and four PET scans (2 WEN and 2 OLY).

Prior to this study the animals were trained to slide out of the water onto
a padded transport mat (Fig.
1). Functional scans (SPECT and PET) were either baseline (no
diazepam prior to ligand injection) or diazepam test scan. For scans under the
influence of diazepam, the animal was given 0.55-0.60 mg kg-1 in a
fish 1 h prior to their removal from the water. Taking a lead from
Mukhametov's observation that diazepam could produce unihemispheric slow waves
(Mukhametov, 1987), we
determined that this amount of diazepam was just over the threshold dosage for
producing signs of unihemispheric EEG slow waves in our dolphins
(Fig. 2). In a separate
preliminary study, the dosage was as determined by EEG telemetry (D70-EEE,
Transoma Medical, Aden Hills, MN, USA) from needle electrodes (25-gauge
Neuroline, Ambu, Denmark) placed to the skull - two electrodes over each
hemisphere while the dolphin was resting quietly, without external stimulation
and with negligible movement, in our veterinary clinic
(Ridgway, 2002). There was an
interval of at least 2 weeks between each diazepam dose to minimize the
animal's potential for developing a tolerance to diazepam.

Each scan procedure began with the dolphin voluntarily sliding onto the
transport mat (Fig. 1). After a
short ride to the dolphin veterinary clinic, the animal was injected with the
ligand [99mTc-bicisate for SPECT,
18F-2-fluoro-2-deoxyglucose (FDG) for PET] into the central
circulation through the common brachiocephalic vein
(Fig. 3) under ultrasound
guidance. After injection of the ligand, the dolphin was kept in a darkened
area and had movement or stimulation minimized during the ligand brain uptake
period. The animal, with trainers and attending veterinarian, was then
transported by covered truck to the nearby imaging facility where the scan was
begun within 2 h after ligand injection.

(A) Ultrasound of the vetrum of the dolphin's neck showing the common
brachiocephalic vein (outlined by arrows) where the ligand was injected (in
the past this vein was sometimes called innominate). (B) Illustration from a
dissection showing the anatomy of the area.

MRI scan of dolphin WEN with a 0.5 T AIRIS II open scanner. The animal
rests on a thin rubber pad. The forward half of the body rests on the scanner
while the rear body rests on a special table constructed for dolphin scans.
Attendants stabilize the animal and keep it wet while the trainer is stationed
in front of the dolphin. A standard human back coil is placed around the
dolphin's head immediately behind the blowhole to cover the area of the
brain.

Since the tables used by the various scanners were not built to take the
mass of a dolphin (180-230 kg), a special table was constructed to fit over
the human table and hold all of the dolphin's mass as the animal was placed in
the scanner. All of the animals were trained to remain still while out of the
water. An animal trainer was present during the scans, stationed just in front
of the animal (Fig. 4). The
animal was kept moist during the scans by sponging its skin with water. The
scanners were protected from the water by placing thin plastic sheeting under
the dolphin. Respiration, body temperature (via rectal thermometer),
and electrocardiogram were monitored during the procedure. Dolphins returned
to their enclosures within 4 h of being removed from the water for the
procedure (Fig. 1).

SPECT scans

Dolphins WEN and FLP were scanned using a SPECT scanner (ADAC Forte SPECT
camera, Milpitas, CA, USA) following an administration of 50 mCi (1850 MBq) of
99mTc-bicisate (Neurolite®), a radiopharmaceutical used to map
blood flow and to diagnose vascular abnormalities of the brain
(Itoh et al., 2001;
Laliberte et al., 2004;
Kusaka et al., 2005). More
details on the scan procedure are published elsewhere
(Houser et al., 2004). In the
control (non-diazepam) scan, the dolphin was not given diazepam until 20 min
after the Neurolite® injection so that the animal was not under the
influence of the diazepam during the radiopharmaceutical uptake period. In the
test scan, the diazepam was given 1 h before the injection of Neurolite®
so that the animal would be under the influence of diazepam while the
Neurolite® was being taken up by the brain. Blood analysis showed
significant levels of circulating diazepam 1 h after oral administration (data
not shown).

(A) Time-of-flight magnetic resonance image (MRI) from dolphin WEN
demonstrating anterior blood flow through arteries of the brain outlined by
the box. (B) Fused MRI and SPECT images from co-registered scans made in the
same dolphin. The colored region corresponds to a reduction in blood flow; the
color bar indicates the relative degree of blood flow reduction with red
indicating maximum reductions in blood flow. The yellow arrow indicates the
central venous sinus. The red arrows indicate the homolog cerebral artery on
the right side of the brain that does not show blood flow reduction.
Registration, image analysis and fusion were performed with ANALYZE.

PET scans

The same non-diazepam/diazepam protocol was followed when dolphins WEN and
OLY were administered 20 mCi (740 MBq) of
18F-2-fluoro-2-deoxyglucose (FDG). FDG is an analog of glucose and
is often used in PET imaging to estimate glucose uptake by the brain. FDG was
given ∼2 h prior to each of four scans (one with and one without prior
diazepam each for WEN and OLY) to map relative metabolic activity within the
brain. As in the SPECT procedure, the animal was kept in a quiet, darkened
room for 20 min after injection of the ligand. The dolphin was then
transported, as outlined above, to the facility where the PET scans were
conducted. Images were acquired on a Seimens HR+ PET scanner (Knoxville, TN,
USA) with the dolphin on the same specially engineered table as used in the
SPECT scan. A 5-min transmission scan was first acquired for attenuation
correction. The emission scan consisted of eight frames of 4 min acquisitions
to allow for repetition in case of any subject movement. This resulted in a
total scan time of approximately 37 min. The scan images were converted from
the ECAT7.2 format to DICOM 3.0 for further processing (see also
Houser et al., 2004).

MRI scan

The first MRI scan ever done on a live dolphin was accomplished with the
dolphin WEN (Figs 4,
5). The dolphin had been
exposed to the recorded sounds of the MRI scanner over 10 training periods
during the month before the actual scan. The dolphin received oral diazepam
(0.55 mg kg-1 body mass) 2 h before the scan. MRI data were
collected on a Hitachi Airis II, 0.5 Tesla (T) scanner. A T2 weighted pulse
sequence was used to acquire image data in the axial plane. Data were acquired
with a slice thickness of 8 mm, a slice interval of 9 mm, and FOV of 280. The
repetition rate (TR) was set to 5700 ms, echo time (TE) set to 125 ms, and
flip angle set to 90°. A total of 20 slices were acquired with a scan time
of approximately 3.5 min. These scan slices were then used for registration of
the SPECT and PET scans.

When a dolphin (MAY), not associated with this project, died of natural
causes, the animal was perfused immediately after death with 4%
paraformaledhyde in buffered ringer's solution. After fixing in situ,
this brain was removed from the skull and scanned on a 3 T scanner for finer
anatomical detail. Based on cranial volume measurements, the brain of MAY was
of similar size to both WEN and OLY. We were not able to MRI scan subject OLY
and thus registered some of the OLY scans to sections of this well-fixed
post mortem brain. Some scans obtained from WEN were also registered
to the MAY scans to show more anatomical details than were available in the
0.5 T scans of WEN.

Image analysis

The Subtraction Ictal SPECT co-registered to MRI algorithm, or SISCOM, was
used to analyze variations in 99mTc-bicisate distribution and FDG
uptake as a function of diazepam induced unihemispheric sleep. The SISCOM
procedure capitalizes on seizure-related transient increases in regional blood
flow to isolate the anatomy of the brain involved in the seizure. The
algorithm is amenable to other methods of assessing variation in brain
function using similar isotopic methods. In this study, SISCOM was employed to
isolate focal regions of the brain that demonstrated reduced blood flow or
reduced metabolism following induction of unihemispheric sleep.

Data acquired from all of the imaging modalities were processed using
Analyze 5.0/6.0, created by the Biomedical Imaging Resource of the Mayo Clinic
(Robb, 1999). All data were
converted to AVW format (native Analyze format) and volumes made cubic
(equivalent voxel dimensions) through the use of linear interpolation. Test
data were co-registered to the control data from the same respective scan type
and animal using the normalized mutual information (NMI) voxel matching
algorithm. The control volume and transformed test volume were then segmented
for creation of binary masks. Using the `Morphology' module of Analyze,
thresholds were applied to the volumes so that isotope activity within the
brain was isolated from surrounding tissues. The volumes were then segmented
and exported as a binary volume. Holes within the binary volumes were filled
utilizing a 2D processing algorithm applied in the transverse, coronal and
sagittal planes, and then once again in the transverse plane. The resultant
control and treatment binary volumes were then multiplied together to form a
binary mask common to the two volumes.

Binary masks common to the SPECT volume were multiplied by the control and
co-registered test volumes, respectively, to generate masked control and
co-registered treatment volumes. The information in these volumes corresponded
only to combined estimates of voxels within the brain. The mean value of all
non-zero voxels was determined for the masked control and masked co-registered
treatment volume and mean values were subsequently used to normalize the
respective volumes to a normalized mean of 100. The normalized co-registered
treatment volume was then subtracted from the normalized control volume,
resulting in a mean voxel value near zero, and the standard deviation of voxel
values within the subtraction volume was calculated.

Four planes from a control FDG PET scan of dolphin WEN registered to a 0.5
T scan of the same animal. No diazepam was given before the ligand injection.
(A) left sagittal section (B) right sagittal section showing the vertex of the
skull (V) and the planes of the axial section (Ax) and coronal section (Cx).
(C) The coronal section showing the left nasal cavity (Ln), the right nasal
cavity (Rn), and the planes of the axial section (Ax), left sagittal section
(Lx), and the right sagittal section (Rx). (D) Axial section. The color bar
indicates relative degree of glucose metabolism with red indicating
maximum.

Voxels corresponding to the brain were segmented from the MRI volume and
the volume passed through an inhomogeneity filter. The control volume was then
co-registered to the segmented MRI volume utilizing the `Surface Matching'
algorithm within Analyze. The registration was fine-tuned through manual
controls and the resultant transform matrix was applied to the control volume,
co-registered treatment volume, and subtraction volume. Once co-registered to
the MRI volume, each of the SPECT and PET volumes were color-mapped to an
8-bit color scale and fused to the MRI to permit the overall pattern of blood
flow or metabolic activity to be observed, as well as activity of focal
regions within the brain to be isolated.

Local reductions in blood flow following diazepam treatment were visualized
by fusing to the MRI only those voxels within the subtraction SPECT volume
with values more than two standard deviations below the mean value of the
subtraction volume. Similarly, local reductions in metabolism were visualized
by fusing to the MRI only those voxels within the subtraction PET volume with
values more than two standard deviations below the mean value of the
subtraction volume. For both the PET and SPECT scans, values more than two
standard deviations below the mean corresponded to a greater than 95%
reduction in isotope distribution and activity, relative to the control. Thus,
the anatomy to which these voxels are mapped correspond to regions of reduced
blood flow (SPECT) and regions of reduced glucose uptake (PET).

Results

Functional SPECT scans

Processing of SPECT images revealed an area of reduced blood flow around a
major artery in the left hemisphere (Fig.
5). The light area at the center of the square on the left shows
the left middle spinal meningeal artery, a major supply to the left brain
(Fig. 5A). On
Fig. 5B are overlaid regions of
decreased blood flow from two previous SPECT scans imaged with 50 mCi (1850
MBq) of technetium (Tc-99m) biscisate (Neurolite®). One SPECT scan was
under the influence of 0.55 mg kg-1 of diazepam while the other was
not. The colored areas show regions of a least two standard deviations of
reduction in blood flow.

Functional PET scans

Pet images from dolphin WEN are shown in Figs
6 and
7. Four sample frames, left and
right sagittal, coronal and axial from Dolphin WEN without diazepam treatment
are shown in Fig. 6. In
Fig. 7 are different coronal,
axial and sagittal sections showing the reduction in glucose consumption in
the diazepam scan in specific areas. In these scan comparisons from Dolphin
WEN, areas of metabolic reduction were most pronounced in the right hemisphere
and especially in the right posterior cortex
(Fig. 7N,O), right insular
cortex (Fig. 7B,M), cerebellum
(Fig. 7B,C,G,N,O), and notably
in the right locus coeruleus (Fig.
7F). However, some areas of marked metabolic reduction appeared in
the left cortex, especially in frontal areas
(Fig. 7A,E).

Discussion

Since this was the first MRI study of a living dolphin, we were concerned
about the animal's potential magnetic sensitivity
(Bauer et al., 1985). The
animal remained quite still while in the magnet and showed no apparent
response during the scan. Examination of the scans revealed no indication of
magnetite; however, since granules of magnetite are usually no more that 50μ
m in diameter the grains, if present, could have been too small to see
with our MRI system.

A subtraction of a diazepam scan from a non-diazepam scan of dolphin WEN
registered to twelve 3.0 T MRI sections of dolphin MAY. From left to right:
coronal (A-C), axial (E-G), left sagittal (I-K) and right sagittal (M-O).
Lines on scan sections (D,H,L,P) at the bottom of each column indicate the
plane of sections from the three scans in the same column above. The color
indicates the relative degree of metabolic reduction in the diazepam scan with
red indicating maximum reductions in glucose consumption. In this series there
is an overall reduction in metabolism in the right brain hemisphere; however,
there are some areas of lower metabolism in the left hemisphere, especially in
frontal areas.

This investigation of functional imaging focused not only on developing
methodology for live dolphin imaging but also on diazepam, known to enhance
sleep in humans and laboratory animals
(Sierra et al., 1997;
Echizenya et al., 2003;
Koop et al., 2004). Diazepam
also produces unihemispheric sleep in dolphins
(Mukhametov, 1984;
Mukhametov, 1987). Our
observations of unihemispheric SWS after diazepam dosages of 0.55 or 0.60 mg
kg-1 body mass is supportive of the previous findings. In the
present study, diazepam caused a reduction in blood flow to one brain
hemisphere as demonstrated by SPECT imaging
(Fig. 5).

The locus coeruleus (LC) is a key structure modulating sleep and
wakefulness in humans and laboratory animals
(Nitz and Siegel, 1997).
Immunohistochemistry has been employed to characterize the dolphin LC
(Manger et al., 2003). There
are no specific specializations in the dolphin LC that set it apart from the
structure of other mammals as might have been expected in a mammal with a
large brain and the ability to sleep unihemispherically. In terrestrial
mammals studied, the firing rate of LC neurons slows during SWS
(Nitz and Siegel, 1997;
Manger et al., 2003). It is
particularly noteworthy that there was a significant reduction in metabolism
of the right LC areas in our study as shown in
Fig. 7F. Our findings lend
support to the suggestion (Manger et al.,
2003) that dolphin LC neurons must fire at a constant rate,
slowing in only one side of the brain during SWS, to maintain muscle tone for
swimming and thermoregulating in cold water.

While it is known that diazepam may cause hypothermia in laboratory mammals
(Dowden et al., 1999),
hypothermia as measured by rectal temperature was not observed in this study.
However, it is possible that regional temperature reductions could be present.
For example one brain hemisphere could be slightly cooler and the other
slightly warmer. Dolphins have numerous retia mirabila that are known to
function as countercurrent heat-exchangers to retain metabolic heat within
certain regions of the body (Rommel et
al., 1993; Heyning and Mead,
1997). The blood supply to the brain comes through a vast retial
network in the dorsum of the thorax not through the internal carotids
(McFarland et al., 1979).

Our studies suggest that cerebral blood flow reduction may be a controlling
factor in the temperature reduction observed by Koval'zon and Mukhametov
during unihemispheric slow wave sleep
(Koval'zon and Mukhametov,
1982). In mammals, brain temperature may be influenced by three
factors: (1) the temperature of blood flowing to the brain, (2) the rate of
cerebral blood flow, and (3) the metabolic heat production of neurons and
glia. Reduced cerebral blood flow and therefore reduced glucose supply likely
will affect regional brain temperature and metabolic heat production.
Furthermore, these factors may impact GABAA receptor sensitivity to
diazepam (Patel et al., 2005;
Garey et al., 1989) such that a
reciprocal effect between the hemispheres could be created so that the active
or `non-sleeping' hemisphere would have a raised threshold for sleep.

Comparison of four sections each of two different scans of dolphin OLY. The
left column (A-D) shows a scan without diazepam while the center column shows
sections from a scan with diazepam (E-H). Overall, metabolism is lower in the
left hemisphere. The color bar indicates the relative degree of glucose
metabolism in sections A-H with red indicating maximum. The right hand column
shows oblique axial scans (as indicated in the upper right, section I) of
dolphin MAY's MRI, to which have been registered the difference volumes
between the two scans. In sections J-L, the colored regions correspond to a
reduction in metabolism in the diazepam scan; the color indicates the relative
degree of metabolic reduction with red indicating maximum reductions in
glucose consumption.

The development of the capability to functionally scan dolphins and the
finding of unihemispheric diazepam effects has suggested a hypothesis of
hemispheric defense. That is, the dolphin brain hemispheres cycle between two
brain states that we will call `State 0' and `State 1.' In `State 0' that
brain hemisphere may be awake and fully alert or it may sleep. The opposite
hemisphere in `State 1' is usually awake and is defended against sleep by
physiological mechanisms as yet not completely understood.

The ability to have EEG slow waves in one brain hemisphere
(Mukhametov et al., 1977;
Mukhametov, 1984;
Mukhametov, 1987;
Ridgway, 2002) while
maintaining an ability to swim and a degree of vigilance
(Lilly, 1964) may not be the
only advantage of the unihemispheric physiology observed in the dolphin brain.
Deep and prolonged diving is important to the foraging success of most dolphin
populations (Evans, 1971;
Ponganis et al., 2003). The
dolphin's large and active brain, especially the huge and elaborate neocortex,
is a considerable metabolic expense
(Robin, 1973;
Hockett, 1978;
McFarland et al., 1979).
Alveolar gas tensions after long dives by dolphins was suggested to indicate
that the dolphin brain might be capable of short periods of anaerobic
metabolism (Ridgway et al.,
1969), a capability lacking, or much reduced, in adult land
mammals that have been studied (anaerobic brain metabolism has been
demonstrated in seals in the later stages of a maximal dive)
(Kerem et al., 1971;
Simon et al., 1974). For the
dolphin, brain oxygen consumption could also be reduced by unihemispheric
vasoconstriction, reduced blood flow and glucose consumption, as observed with
our SPECT and PET scans. The ability to partially `shut down' or at least
reduce oxygen and glucose consumption in a major portion of the brain might be
an advantage to a dolphin making repetitive, prolonged feeding dives.

This study has shown that dolphins can be trained to participate in
non-invasive scans that can be useful in understanding their brain blood flow,
metabolism and many other aspects of their specialized physiology and anatomy.
Functional scans may help to elucidate the degree of laterality of sensory and
motor systems. Scans may reveal neurotransmitter or molecular mechanisms of
physiology that cannot be explored in any other way. The techniques developed
here can also be useful in detecting pathology and in the clinical care of
these interesting and valuable animals.

ACKNOWLEDGEMENTS

All experiments were conducted in accordance with a protocol approved by
the Institutional Animal Care and Use Committees of the Navy Marine Mammal
Program, Space and Naval Warfare Systems Center, San Diego, CA, USA and the
School of Medicine, University of California, San Diego. We thank J. Corbeil
and the staff of Molecular Imaging Incorporated and the Department of Nuclear
Medicine at the University of California, San Diego Medical Center for their
assistance in performing scans. We also thank the Biomedical Imaging Resource
of the Mayo Clinic, Rochester for their assistance with ANALYZE. Drs Eric
Jensen and Chris Dold, Joel Baumbaugh (Health Physics) and the animal care and
training staff of the Space and Naval Warfare Systems Center (SSC San Diego),
were critical for their training, transport and care of the dolphin subjects.
We especially thank Mark Todd and Tricia Kamolnick for their technical
expertise in training and their continuous efforts with the dolphin subjects.
We appreciate the encouragement of Drs John Carney, Lisa Eley, Amy Kruse and
Brett Giroir. We also thank two anonymous reviewers who made very helpful
suggestions. This project supported by the Defense Advanced Research Projects
Agency (DARPA) under SSC San Diego Contract N66001-05-C-0040.

Ridgway, S. H. (2002). Asymmetry and symmetry
in brain waves from dolphin left and right hemispheres: some observations
after anesthesia, during quiescent hanging behavior, and during visual
obstruction. Brain Behav. Evol.
60,265
-274.

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